The functionality of permanent magnet direct current brush-commutated electric motors requires switching current from winding to winding. As the brushes slide from one bar to another an arching occurs due to the back electromotive force (EMF) generated by the stored electromagnetic energy in the current carrying coils. The arching generates broadband electrical noise over a wide frequency spectrum. The spectrum energy distribution is influenced by switching of coils (commutation) that is proportional to the motor speed.
There are two modes of conducted noise generated in a motor, differential mode and common mode. The differential mode filtering can be accomplished with a shunt capacitor connected across the positive and negative motor terminals and use of series inductors or ferrite beads (chokes) to reduce radio frequency (RF) noise currents. The common mode filtering is also accomplished by including series impedance in both leads in addition to referencing the shunt capacitor and the negative terminal to case. However, the motor case and vehicle ground must be at the same potential. If the case and vehicle ground are at different potential then the RF reference can be provided by an additional capacitor. This type of filtering is shown in FIGS. 1, 2, and 3.
FIG. 1 shows a conventional brush card assembly 10 for a two brush, four pole (permanent magnet) motor with a link wound commutator. In this case, the Radio Frequency Interference (RFI) filtering is realized by placing a capacitor 11 between the positive terminal bar 12 and negative terminal bar 14 that are directly connected to the respective chokes. The positive choke 16 is connected to the positive brush inside brush tube 18 via a shunt wire 20 and the negative choke 22 is connected to the negative brush inside brush tube 24 via shunt wire 26. Furthermore, a motor case/stator assembly is connected to the negative terminal bar 14 through the ground shunt wire 28. The ground shunt wire 28 is at the same potential as the vehicle ground. In this case, the terminal bars are insert molded into a connector body 30.
FIG. 2 shows a conventional brush card assembly 10′ for a two brush, four pole (permanent magnet) motor with a link wound commutator. In this case, RFI filtering is realized by placing a capacitor 32 between the positive terminal bar 16 and motor case/stator assembly and another capacitor 34 between the negative terminal bar 14 and the motor case/stator assembly. Therefore, the motor case has the same potential as the vehicle ground. The terminal bars are directly connected to the respective chokes (the positive choke 16 is connected to the positive brush 18 and the negative choke 22 is connected to the negative brush 24 in the manner discussed above with regard to FIG. 1). In this case, the positive lead wire 36 and the negative lead wire 38 are attached to the terminal bars and the assembly is then over molded in a grommet 40.
FIG. 3a shows a conventional brush card assembly for a four brush, four pole (permanent magnet) motor. In this case, the RFI filtering is a bit more complex because there are four brushes used (two positive 18, 18′ and two negative 24, 24′) and there is a choke 37 connected between each brush and the respective power input terminal bar. As shown in FIG. 3b, two chokes are not seen since they are on the other side of the brush card. The capacitor 42 is connected between the positive terminal input bar 44 and the negative input terminal bar 46. The capacitor 48 is connected between the positive input terminal bar 44 and motor case/stator assembly. Also, the motor case/stator assembly has the same potential as the vehicle ground through the ground shunt wire 50. In this case, similar to the brush card in FIG. 2, the lead wires 36, 38 are attached to the terminal bars and the assembly is then over molded in a grommet.
FIG. 4 shows another conventional brush card assembly with four brushes for a four pole (permanent magnet) motor. However there are only two chokes used, one for both positive brushes and the other one for both negative brushes. The choke 52 is connected between the negative input terminal bar 46 and the brush 54 (negative brush). The choke 56 is connected between the positive input power terminal bar 44 and brush 58 (positive brush). The capacitor 60 is connected between the positive and negative input terminal bars. The capacitor 62 is connected between the positive input terminal bar and motor case/stator assembly. In this case, the negative terminal bar 46 has an extended feature 64 (instead of the ground shunt wire as shown in FIGS. 1, 2, and 3)) to make contact to the motor case/stator assembly. Therefore, the motor case/stator assembly still has the same polarity as the vehicle ground.
Although the construction of the brush card assembly for a four-brush configuration is significantly different from the two-brush configuration, the method of assembling the capacitors and grounding to motor case for RFI suppression are the same. The attachment of capacitors 32, 48, and 62 in FIGS. 2, 3 and 4, respectively, and capacitor 34 in FIG. 2 is manual labor intensive. Furthermore, paint must be removed (grinded off) from the stator/motor case at the contact area of ground strap shunt wire or the ground terminal bar in order to ensure proper grounding.
Therefore, a more universal and versatile terminal bar system is needed that can be more cost effective and that can accommodate automated assembly.